M. Drechsler’s research while affiliated with French National Centre for Scientific Research and other places

A combination of field emission (FEM) and transmission electron microscopy (TEM) is used to measure the anisotropy of the surface tension (γ) of a metal (nickel). The stationary form of a clean nickel tip is produced and controlled in a field electron microscope, under ultrahigh vacuum. The shape of the tip is visualized in a TEM and then analysed. It is shown that the cap around the apex of a bulbous tip closely approximates the equilibrium shape of a nickel crystal. By using the inverse Wulff construction, the anisotropy of γ can be measured as a function of crystallographic orientation. Values of γ normalized to (111) have been obtained for the 〈200〉 and the 〈2̄20〉 zones. The maximum anisotropy is found at [026] along the 〈200〉 zone (). The results agree with the available experimental or theoretical data reasonably well.

The surface diffusion of palladium on the curved part of a tungsten crystal is studied by field electron microscopy. The variation of the local coverage distribution is measured by a probe-hole device on the stepped surface region around (001). The measured data allow a determination of the mass transport surface diffusion coefficient D of Pd on W across atomic steps as a function of temperature, coverage and step density. D has been found (1) to be constant for a given step density and for coverages lower than about 5 × 1014 Pd adatoms/cm2, (2) to increase for higher coverages, and (3) to increase with increasing step density for a given coverage. The activation energy of the process is nearly constant (about 24 kcal/mol) for all coverages up to about 6 × 1014 adatoms/cm2, while the pre-exponential factor of D increases with increasing step density. Interpretation of the results gives some information on the diffusion mechanism.

The surface self-diffusion on crystals is composed of different types of atom displacements as those on terraces or across atomic steps. The field ion microscope enables a visualization of the displacement of individual atoms on a metal tip, but unfortunately this is limited to terraces and only to those adatoms which arrive from the vapor phase and not from the crystal itself.

The influence of interactions between adsorbed particles on their diffusion constant is investigated by kinetic Ising models with independent nearest neighbour hops. This leads to expressions for the ratio of the diffusion constant at arbitrary coverage θ to its value at θ = 0 as a function of the interaction energies relative to temperature. It is shown that under certain conditions this quantity obeys a particle-hole symmetry. Exact results in the whole range of densities are given in one dimension for nearest neighbour interaction. They already yield a qualitative agreement with experimental results and are also compared to corresponding numerical simulations. The introduction of a next nearest neighbour interaction is shown to produce drastic changes in the density dependence of the diffusion constant in some of the cases. A generalized quasichemical approximation and a virial expansion are made in two dimensions, leading to a better agreement with the measurements.

A field electron probe-hole technique is used to determine local adsorption coverages on a tip. This enables the determination of coverage distributions of an adsorbate (palladium) in thermodynamic equilibrium along a curved crystal surface (W zone between {100} and {310}). Such equilibria show local coverage variations with the local step density. An interpretation of the measured distributions gives information about the relative energies of the different adsorption sites.

Field electron microscopy is used to study the surface diffusion of lead on tungsten. A simple method to measure rough values of the diffusion coefficient and its dependence on sub-monolayer coverage is described and tested. In the region around (001) the displacement energy found is about 1.30 eV/atom up to 1015 atoms/cm2 where it decreases to 0.78 eV/atom. In the residual region except (110) this energy at 1.5×1014 atoms/cm2 is 1.22 eV/atom, it decreases at 4 × 1014 atoms/cm2 to 0.61 eV/atom and increases at 1015 atoms/cm2 to 0.78 eV/atom. Corresponding values of the diffusion coefficient D and of the preexponential D0 are given. The dependence of D on submonolayer coverage is discussed.

The influence of a coadsorbed submonolayer (carbon) on the diffusion of adatoms (lead) along a crystal surface (tungsten) is studied in a preliminary manner by a field electron microscope technique. Experiments show that the surface diffusion of lead is strongly affected by the coadsorption of carbon: (1) The diffusion of Pb which is easy around (110) and (111) and difficult around (100) on clean tungsten becomes easier around (100) than around (110) and (111). (2) The diffusion anisotropy on the stepped surface around (100) is changed by carbon adsorption. (3) C adatoms enhance the diffusion of Pb especially around (100). (4) The mean carbon coverage required to observe these effects is only a few hundredths of a monolayer. The dependence of the surface diffusion on such small impurity coverages is discussed.

Dendritic gold crystallites on graphite are heated in ultra high vacuum up to less than 0.5 of the melting point (Tm). Electron microscopy shows that the gold crystallites change their shapes by surface self-diffusion. The dendritic contours round off while the crystallite remains very flat (20 to 40 Å). The increase with time of the radii of dendrite tips is measured statistically. Such an evolution can be described by analogy to the blunting of either metal tips (Nichols and Mullins) or monoatomic cleavage tips (Höche and Bethge). Using this result, a new technique to measure surface self-diffusion coefficients (D) is proposed. Test measurements have shown that this is an interesting, very sensitive method to measure D (down to 10−13 cm−2 s−1) which enables measurements to be made in an unusual low temperature range (0.25 Tm < T < 0.5Tm). In special cases the dendrites are split by the surface self-diffusion which is qualitatively in agreement wih the theory.